Thousands of snowballs rolled in a flat central Idaho field look like the work of hundreds of go-getting kids. But there were no human tracks.

A rare weather event caused the spontaneous snowballs. They appeared at the Nature Conservancy's Silver Creek Preserve and surrounding fields. The area is near the tiny town of Picabo.

Sunny Healey is the preserve manager. She spotted the cylindrical shapes up to 18 inches high on Jan. 30. She saw them following an overnight windstorm. They created long lines in the snow as they moved.

"You could see the tracks that they made. And I thought that was really curious," Healey said. "I had to stop a couple times. Then, along Highway 20, there were thousands of them."

So-called snow rollers are rare. The exact weather conditions needed to form them are not defined, said Jay Breidenbach. He is a meteorologist with the National Weather Service. Snow rollers up to 18 inches are really rare.

"Those are some pretty big rollers," Breidenbach said. "I've seen some small rollers. But never that big."

In general, it takes an odd combination of a couple of inches of snow with the right water density and temperatures near freezing, followed by strong winds, he said.

"It can't be real dry snow or it would blow into drifts," Breidenbach said.

Rollers need some type of firmer base, such as a frozen layer of earlier snow, for the new powder to start rolling on. Plus, the wind must be strong and steady. But not with powerful gusts that could damage the formations.

"It would probably blow them apart because they are fragile," Breidenbach said.

It snowed on Jan. 29. The snow became wetter toward evening, Healey said. She lives at the preserve. She said the winds woke her up.

In her five years working at the preserve, she had never seen such an event. But a local rancher in his 70s told her he's spotted them twice.

"We know basically how they form and why they form. But we don't know the exact details," Breidenbach said. "It would be interesting to go there with some weather instruments to watch them form."

Because the snow was firm and frozen layers under that so the wind just blew and they rolled up and formed spontaneous snowballs.

elliew-bel

2/25/2016 - 02:00 p.m.

It had the water density, and temperature, near freezing, and strong winds. Also the wind must be strong and steady.

sophies-bel

2/25/2016 - 02:02 p.m.

The wind blew so hard and the snow was slightly wet that wind could blow snowballs.

sarahj-bel

2/25/2016 - 02:04 p.m.

Wind created the snowballs by having frozen layer then a earlier layer of snow but the temperature has to be just right and the wind has to blow right because the snow balls are fragile.

jocelynr1-nor

3/01/2016 - 09:56 a.m.

The wind is so hard that it rolled snow into a big snowball.

rogerele1-dil

3/01/2016 - 02:48 p.m.

Earlier snow was frozen and fresh snow came together

brianna-rya

3/12/2017 - 09:33 p.m.

The wind blews so hard that it makes the snow falls down. And that make snowballs because it rolls down a hell that has snow on it.The forse from the wind makes the snowballs get biger and biger. so that is how the wind makes snowballs.

sophiep1-rya

3/12/2017 - 09:34 p.m.

Because the snow was firm and frozen layers under thst so the just blew and they rolled up and formed spontaneous snowballs.

elias-rya

3/12/2017 - 09:37 p.m.

the snow and the ice freeze to gether and combinsdinds and becomes hard snow

ameliam1-rya

3/12/2017 - 10:03 p.m.

Wind creates snowballs by the snow being firm and frozen layers under that so the wind blew. By them rolling together they formed snowballs.